Scalable Magnetic Random Access Memory Device

ABSTRACT

A magnetic memory cell is provided. The magnetic memory cell includes at least one fixed magnetic layer and a plurality of free magnetic layers, separated from the at least one fixed magnetic layer by at least one barrier layer. The free magnetic layers include a first free magnetic layer adjacent to the barrier layer, a second free magnetic layer separated from the first free magnetic layer by at least one spacer layer, and a third free magnetic layer separated from the second free magnetic layer by at least one anti-parallel coupling layer. A magnetic moment of the first free magnetic layer is greater than both a magnetic moment of the second free magnetic layer and a magnetic moment of the third free magnetic layer. The magnetic memory cell may be used in conjunction with a magnetic random access memory device.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.11/481,541 filed on Jul. 6, 2006, the disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to data storage, and more particularly, todata storage devices and techniques for use thereof.

BACKGROUND OF THE INVENTION

Semiconductor devices, such as magnet random access memory (MRAM)devices, use magnetic memory cells to store information. Information isstored in the magnetic memory cells as an orientation of themagnetization of a free layer in the magnetic memory cell as compared toan orientation of the magnetization of a fixed (e.g., reference) layerin the magnetic memory cell. The magnetization of the free layer can beoriented parallel or anti-parallel relative to the fixed layer,representing either a logic “1” or a logic “0.” The orientation of themagnetization of a given layer (fixed or free) may be represented by anarrow pointing either to the left or to the right. When the magneticmemory cell is sitting in a zero applied magnetic field, themagnetization of the magnetic memory cell is stable, pointing eitherleft or right. The application of a magnetic field can switch themagnetization of the free layer from left to right, and vice versa, towrite information to the magnetic memory cell.

One of the objectives of MRAM is to have a low operating power and smallarea. This objective requires a low switching field for the magneticmemory cell, because a low switching field uses a low switching current,which uses less power, and because smaller currents require smallerswitches, which occupy less space. However, as the area of the magneticmemory cells becomes increasingly smaller, a process referred to as“scaling” due to the fact that the area of the magnetic memory cell isscaled down to allow for more magnetic memory cells in the same area,the switching field actually increases.

U.S. Pat. No. 6,545,906 issued to Savtchenko, et al., entitled “Methodof Writing to Scalable Magnetoresistance Random Access Memory Element”(hereinafter “Savtchenko”) describes a toggle free layer for use in MRAMdevices. Prior to Savtchenko, MRAM devices employed a single free layerdesign. Both of these approaches, however, are prone to the problemsassociated with scaling that are described above. Namely, as the size ofthe magnetic memory cell is scaled down, an increased amount of power isrequired to switch the magnetic memory cell. For example, as much as 80Oersteds (Oe) can be required to switch a 150 nanometer (nm) togglemagnetic memory cell.

Thus, scalable magnetic memory devices having reduced switching fieldswould be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for reducing the switchingfields in magnetic memory devices. In one aspect of the invention, amagnetic memory cell is provided. The magnetic memory cell comprises atleast one fixed magnetic layer and a plurality of free magnetic layers,separated from the at least one fixed magnetic layer by at least onebarrier layer. The free magnetic layers include a first free magneticlayer adjacent to the barrier layer, a second free magnetic layerseparated from the first free magnetic layer by at least one spacerlayer, and a third free magnetic layer separated from the second freemagnetic layer by at least one anti-parallel coupling layer. A magneticmoment of the first free magnetic layer is greater than both a magneticmoment of the second free magnetic layer and a magnetic moment of thethird free magnetic layer.

In another aspect of the invention, a magnetic random access memory(MRAM) device is provided. The MRAM device comprises a plurality of wordlines oriented orthogonal to a plurality of bit lines, and a pluralityof magnetic memory cells configured in an array between the word linesand bit lines. At least one of the plurality of magnetic memory cellscomprises at least one fixed magnetic layer, and a plurality of freemagnetic layers, separated from the at least one fixed magnetic layer byat least one barrier layer. The free magnetic layers include a firstfree magnetic layer adjacent to the barrier layer, a second freemagnetic layer separated from the first free magnetic layer by at leastone spacer layer, and a third free magnetic layer separated from thesecond free magnetic layer by at least one anti-parallel coupling layer.A magnetic moment of the first free magnetic layer is greater than botha magnetic moment of the second free magnetic layer and a magneticmoment of the third free magnetic layer.

In yet another aspect of the invention, a method of writing data to anMRAM device having a plurality of word lines oriented orthogonal to aplurality of bit lines, and a plurality of magnetic memory cellsconfigured in an array between the word lines and bit lines, comprisesthe following steps. A word line current is provided to a given one ofthe word lines to select all of the magnetic memory cells along thegiven word line. At least one of the selected magnetic memory cellscomprises at least one fixed magnetic layer and a plurality of freemagnetic layers, separated from the at least one fixed magnetic layer byat least one barrier layer. The free magnetic layers include a firstfree magnetic layer adjacent to the barrier layer, a second freemagnetic layer separated from the first free magnetic layer by at leastone spacer layer, and a third free magnetic layer separated from thesecond free magnetic layer by at least one anti-parallel coupling layer.A magnetic moment of the first free magnetic layer is greater than botha magnetic moment of the second free magnetic layer and a magneticmoment of the third free magnetic layer. A bit line current is providedto each of the bit lines corresponding to the selected magnetic memorycells. The word line current is removed. The bit line current isremoved.

In still another aspect of the invention, a magnetic memory cell isprovided. The magnetic memory cell comprises at least one fixed magneticlayer, and a plurality of free magnetic layers, separated from the atleast one fixed magnetic layer by at least one barrier layer. The freemagnetic layers include a first free magnetic layer adjacent to thebarrier layer, a second free magnetic layer separated from the firstfree magnetic layer by at least one first parallel coupling layer, and athird free magnetic layer separated from the second free magnetic layerby at least one second parallel coupling layer. A magnetic moment of thesecond free magnetic layer is greater than both a magnetic moment of thefirst free magnetic layer and a magnetic moment of the third freemagnetic layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary magnetic memory cellaccording to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary magnetic memory cell arrayaccording to an embodiment of the present invention;

FIGS. 3A-B are diagrams illustrating top-down views of stable magneticstates of the exemplary magnetic memory cell of FIG. 1 in a zero appliedmagnetic field according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating another exemplary magnetic memory cellaccording to an embodiment of the present invention;

FIGS. 5A-B are diagrams illustrating top-down views of stable magneticstates of the exemplary magnetic memory cell of FIG. 4 in a zero appliedmagnetic field according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating an exemplary methodology for writingdata to a magnetic memory cell array according to an embodiment of thepresent invention;

FIGS. 7A-B are graphs representing current pulse sequences used to writedata to an individual magnetic memory cell according to an embodiment ofthe present invention;

FIG. 8 is a graph illustrating an exemplary critical switching curve fora magnetic memory cell according to an embodiment of the presentinvention;

FIGS. 9A-D are graphs illustrating magnetic moment rotations in magneticlayers of a magnetic memory cell according to an embodiment of thepresent invention;

FIGS. 10A-B are graphs illustrating moment rotations for a magneticmemory cell when a field is applied in an easy axis direction accordingto an embodiment of the present invention; and

FIGS. 11A-B are graphs illustrating moment rotations for a magneticmemory cell when a field is applied in a hard axis direction accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating exemplary magnetic memory cell 100.Magnetic memory cell 100 comprises tunnel barrier 106 deposited on fixedlayer 104, and free layer 102 deposited on tunnel barrier 106.

According to the exemplary embodiment depicted in FIG. 1, magneticmemory cell 100 can be configured, i.e., patterned, to have, when viewedfrom top-down view 126, a circular (or elliptical) shape. See also thetop-down depictions of magnetic memory cell 100 having a circular shapein FIGS. 3A-B, described below. It is to be understood, however, thatany other patternable shapes and/or configurations can be employed inaccordance with the present teachings. As will be described in detailbelow, magnetic memory cell 100 may be used in conjunction with amagnetic random access memory (MRAM) device.

As shown in FIG. 1, free layer 102 is a multiple-layer structure.Namely, free layer 102 comprises magnetic layer 108 adjacent to tunnelbarrier 106. Spacer layer 114 is deposited on a side of magnetic layer108 opposite tunnel barrier 106. Magnetic layer 112 is deposited on aside of spacer layer 114 opposite magnetic layer 108. Anti-parallelcoupling (AP-coupling) layer 110 is deposited on a side of magneticlayer 112 opposite spacer layer 114. Finally, magnetic layer 116 isdeposited on a side of AP-coupling layer 110 opposite magnetic layer112.

Each of magnetic layers 108, 112 and 116 can comprise any suitableferromagnetic material, including, but not limited to, one or more of anickel/iron alloy (NiFe alloy) and alloys containing one or more of Ni,Fe, Cobalt (Co) and a rare earth element. Further, any one of magneticlayers 108, 112 and 116 can have a same, or a different, composition asany of the other magnetic layers 108, 112 and 116. For example,according to one exemplary embodiment, all three magnetic layers 108,112 and 116 comprise the same NiFe alloy.

Each of magnetic layers 108, 112 and 116 has a magnetic moment μassociated therewith, wherein μ equals the product of a magnetization ofthe layer M_(s), a thickness of the layer T and an area of the layer A,i.e.,

μ=M _(s) ·T·A.  (1)

As will be described below, μ points in a particular direction, i.e., adirection of magnetization. Magnetic layers 108, 112 and 116 havemagnetic moments μ₁, μ₂, and μ₃, respectively, associated therewith.According to the present teachings, μ₁ is greater than both μ₂ and μ₃,i.e., μ₁>μ₂ and μ₁>μ₃. Further, according to one exemplary embodiment,μ₂ is equal to μ₃, i.e., μ₂=μ₃.

When magnetic layers 108, 112 and 116 are configured to have a circularshape, as described above, it can be assumed that each of magneticlayers 108, 112 and 116 occupies the same area, i.e., magnetic layers108, 112 and 116 have areas A₁, A₂ and A₃, respectively, associatedtherewith wherein A₁=A₂=A₃. Further, when magnetic layers 108, 112 and116 have the same composition, as described above, it can be assumedthat each of magnetic layers 108, 112 and 116 has the samemagnetization, i.e., magnetic layers 108, 112 and 116 havemagnetizations M_(s1), M_(s2) and M_(s3), respectively, associatedtherewith wherein M_(s1)=M_(s2)=M_(s3). Accordingly, when both of theabove conditions apply, i.e., when magnetic layers 108, 112 and 116 arecircular and each layer has the same composition, T is the sole variablein μ differences between the layers. According to this exemplaryembodiment, magnetic layer 108 is adapted to have a greater thicknessthan either magnetic layer 112 or magnetic layer 116, i.e., magneticlayers 108, 112 and 116 have thicknesses T₁, T₂ and T₃, respectively,associated therewith wherein T₁>T₂ and T₁>T₃. See, for example, FIG. 1.Further, according to one exemplary embodiment, magnetic layer 112 hasthe same thickness as magnetic layer 116, i.e., T₂=T₃.

Magnetic layers 108 and 112 are separated by spacer layer 114. Spacerlayer 114 mediates zero coupling between magnetic layers 108 and 112.According to an exemplary embodiment, spacer layer 114 comprises one ormore of tantalum nitride (TaN), Ta, titanium (Ti), tungsten (W) andniobium (Nb). Materials that can give zero coupling, such as ruthenium(Ru), can also be used if the thickness of the layer is adjusted to givezero coupling.

Magnetic layers 112 and 116 are separated by AP-coupling layer 110.AP-coupling layer 110 mediates anti-parallel magnetic coupling betweenmagnetic layers 112 and 116. According to an exemplary embodiment,AP-coupling layer 110 comprises one or more of Ru, osmium (Os), copper(Cu), chromium (Cr), molybdenum (Mo), rhodium (Rh), rhenium (Re) andiridium (Ir).

As described above, tunnel barrier 106 separates free layer 102 fromfixed layer 104. Tunnel barrier 106 comprises any suitable tunnelbarrier material, including, but not limited to, one or more of aluminumoxide (AlOx) and magnesium oxide (MgO).

As shown in FIG. 1, fixed layer 104 can also be a multiple-layerstructure. Namely, fixed layer 104 comprises magnetic layer 118 adjacentto tunnel barrier 106 and separated from magnetic layer 120 byAP-coupling layer 122. Anti-ferromagnetic (AF) layer 124 is adjacent toa side of magnetic layer 120 opposite AP-coupling layer 122.

Each of magnetic layers 118 and 120 can comprise any suitableferromagnetic material, including, but not limited to, one or more of analloy containing Ni, Fe or Co, and can have a same, or a different,composition as the other of magnetic layers 118 and 120. AP-couplinglayer 122 mediates anti-parallel magnetic coupling between magneticlayers 118 and 120, and can comprise one or more of Ru, Os, Cu, Cr, Mo,Rh, Re and Ir.

AF layer 124 serves to pin the direction of magnetization of magneticlayers 118 and 120. AF layer 124 can comprise a metal alloy, including,but not limited to, one or more of a platinum/manganese alloy (PtMn) andan iridium/manganese alloy (IrMn).

While FIG. 1 depicts magnetic memory cell 100 as having tunnel barrier106 being deposited on top of fixed layer 104 and free layer 102 beingdeposited on top of tunnel barrier 106, this configuration is merelyexemplary, and other configurations can be employed. By way of exampleonly, the layers may be deposited in the reverse order to that shown inFIG. 1, i.e., with tunnel barrier 106 being deposited on top of freelayer 102 and fixed layer 104 being deposited on top of tunnel barrier106, so long as the free magnetic layer having the greatest magneticmoment (e.g., magnetic layer 108) is adjacent to tunnel barrier 106.

FIG. 2 is a diagram illustrating exemplary magnetic memory cell array200. Magnetic memory cell array 200 can be employed as an MRAM device.

Magnetic memory cell array 200 comprises bit lines 202 and word lines204 running orthogonal to each other above and below a plurality ofmagnetic memory cells 100. The configuration of magnetic memory cellarray 200 shown in FIG. 2 is merely exemplary, and other configurationsare possible. By way of example only, magnetic memory cell array 200 canbe configured to have bit lines 202 run below magnetic memory cells 100and word lines 204 run above magnetic memory cells 100.

Methods for writing data to magnetic memory cell array 200 will bedescribed in detail below. In general, however, each word line 204applies a magnetic field H_(word) along a y-axis of each magnetic memorycell 100, and each bit line 202 applies a magnetic field H_(bit) alongan x-axis of each magnetic memory cell 100. The y-axis comprises a hardswitching axis of each magnetic memory cell 100 and the x-axis comprisesan easy switching axis of each magnetic memory cell 100.

FIGS. 3A-B are diagrams illustrating top-down views of stable magneticstates of exemplary magnetic memory cell 100 in a zero applied magneticfield. Arrows 302, 304 and 306 represent magnetic moments μ₁, μ₂ and μ₃of magnetic layers 108, 112 and 116, respectively. Since μ₁>μ₂ andμ₁>μ₃, as described above, a heavier arrow is used for arrow 302 thanfor arrows 304 and 306. According to the present teachings, at leastmagnetic layer 108 will have an intrinsic anisotropy set along thex-axis direction (i.e., along the easy switching axis). Further, byvirtue of the fact that μ₁>μ₂ and μ₁>μ₃, μ₁ will point along theintrinsic anisotropy axis of magnetic layer 108. As such, there are twostable magnetic states for magnetic memory cell 100 when in a zeroapplied magnetic field. The first stable magnetic state is when μ₁points in the positive x-axis direction. See, for example, FIG. 3Awherein arrow 302 is pointing to the right. The second stable magneticstate is when μ₁ points in the negative x-axis direction. See, forexample, FIG. 3B wherein arrow 302 is pointing to the left.

The dipole fields from each of magnetic layers 108, 112 and 116 make μ₁,μ₂ and μ₃ spread out in a triangle. See FIGS. 3A-B wherein arrows 302,304 and 306 spread out in a triangle.

As shown in FIG. 3A, when arrow 302 (μ₁) points to the right, arrow 304(μ₂) points to the lower left and arrow 306 (μ₃) points to the upperleft. Similarly, as shown in FIG. 3B, when arrow 302 (μ₁) points to theleft, arrow 304 (μ₂) points to the upper right and arrow 306 (μ₃) pointsto the lower right.

It is notable that in the two stable magnetic states shown in FIGS. 3Aand 3B, it is immaterial which of μ₂ or μ₃ points to the upper/lowerleft/right, respectively. For example, the operation of magnetic memorycell 100 would not change if in the configuration shown in FIG. 3B arrow304 (μ₂) pointed to the lower right instead of the upper right and arrow306 (μ₃) pointed to the upper right instead of the lower right.

FIG. 4 is a diagram illustrating exemplary magnetic memory cell 400.Magnetic memory cell 400 comprises tunnel barrier 406 deposited on fixedlayer 404, and free layer 402 deposited on tunnel barrier 406.

Free layer 402 is a multiple-layer structure. Namely, free layer 402includes three magnetic layers 408, 412 and 416, each of which cancomprise a ferromagnetic material, including, but not limited to, one ormore of a NiFe alloy and alloys containing one or more of Ni, Fe, Co anda rare earth element. Further, any one of magnetic layers 408, 412 and416 can have a same, or a different, composition as any one of the othermagnetic layers 408, 412 and 416.

Magnetic layer 412 is separated from, and is parallel coupled(P-coupled) to both magnetic layers 408 and 416 by P-coupling layers 410and 414, respectively. Each of P-coupling layers 410 and 414 cancomprise any suitable P-coupling materials, including, but not limitedto, one or more of Ru, Os, Cu, Cr, Mo, Rh, Re and Ir, and may have asame, or a different, composition as each other.

Each of magnetic layers 408, 412 and 416 has a magnetic moment μassociated therewith. The value of μ can be calculated according toEquation 1, above. Magnetic layers 408, 412 and 416 have magneticmoments μ₄, μ₅ and μ₆, respectively, associated therewith. According tothe present teachings, μ₅ is greater than μ₄ and μ₆, i.e., μ₅>μ₄ andμ₅>μ₆. Further, according to one exemplary embodiment, μ₄ is equal toμ₆, i.e., μ₄=μ₆. As described in detail above, if magnetic layers 408,412 and 416 occupy the same area A and have the same magnetizationM_(s), then layer thickness T is the sole variable in moment μdifferences between the magnetic layers. Such an instance is depicted inFIG. 4, wherein magnetic layer 412 is shown to have a greater thicknessthan either of magnetic layers 408 and 416.

Fixed layer 404 is a multiple-layer structure. Namely fixed layer 404comprises magnetic layer 418 adjacent to tunnel barrier 406 andseparated from magnetic layer 420 by AP-coupling layer 422. AF layer 424is adjacent to a side of magnetic layer 420 opposite AP-coupling layer422. Magnetic layers 418 and 420 can have a same, or a different,composition as each other, and can comprise any suitable ferromagneticmaterial, including, but not limited to, one or more of a NiFe alloy andalloys containing one or more of Ni, Fe, Co and a rare earth element.AP-coupling layer 422 mediates anti-parallel magnetic coupling betweenmagnetic layers 418 and 420 and can comprise one or more of Ru, Os, Cu,Cr, Mo, Rh, Re and Ir.

AF layer 424 serves to pin the direction of magnetization of magneticlayers 418 and 420. AF layer 424 can comprise a metal alloy, including,but not limited to, one or more of a PtMn alloy and an IrMn alloy.

A plurality of magnetic memory cells 400 may be used in a magneticmemory cell array. For example, a plurality of magnetic memory cells 400can be used in magnetic memory cell array 200, described in conjunctionwith the description of FIG. 2 above, in place of magnetic memory cells100.

FIGS. 5A-B are diagrams illustrating top-down views of stable magneticstates of exemplary magnetic memory cell 400 in a zero applied magneticfield. Arrows 502, 504 and 506 represent magnetic moments μ₄, μ₅ and μ₆of magnetic layers 408, 412 and 416, respectively. Since μ₅>μ₄ andμ₅>μ₆, as described above, a heavier arrow is used for arrow 504 thanfor arrows 502 and 506. According to the present teachings, at leastmagnetic layer 412 will have an intrinsic anisotropy set along thex-axis direction (i.e., along the easy switching axis). Further, byvirtue of the fact that μ₅>μ₄ and μ₅>μ₆, μ₅ will point along theintrinsic anisotropy axis of magnetic layer 412. As such, there are twostable magnetic states for magnetic memory cell 400 in a zero appliedmagnetic field. The first stable magnetic state is when μ₅ points in thepositive x-axis direction. See, for example, FIG. 5A wherein arrow 504points to the right. The second stable magnetic state is when μ₅ pointsin the negative x-axis direction. See, for example, FIG. 5B whereinarrow 504 is pointing to the left.

As shown in FIG. 5A, when arrow 504 (μ₅) points to the right, arrow 502(μ₄) points to the lower left and arrow 506 (μ₆) points to the upperleft. Similarly, as shown in FIG. 5B, when arrow 504 (μ₅) points to theleft, arrow 502 (μ₄) points to the upper right and arrow 506 (μ₆) pointsto the lower right. As explained above, in FIGS. 5A-B it is immaterialwhich of μ₄ or μ₆ points to the upper/lower left/right, respectively.

FIG. 6 is a diagram illustrating exemplary methodology 600 for writingdata to a magnetic memory cell array, such as magnetic memory cell array200, described in conjunction with the description of FIG. 2, above,having a plurality of word lines oriented orthogonal to a plurality ofbit lines and a plurality of magnetic memory cells (e.g., magneticmemory cells 100 or magnetic memory cells 400, both described above)therebetween. As described above, in each magnetic memory cell in thearray, the free magnetic layer having the greatest moment μ has anintrinsic anisotropy pointing along the x-axis direction and thusaligned with the direction of field applied by the bit line.

In step 602, a current is passed along a given one of the word lines (aword line current) thereby selecting all of the magnetic memory cells onthat given word line (i.e., the word line current destabilizes themagnetic memory cells, which essentially erases all pre-existing dataand makes the magnetic memory cells easier to write). Namely, all of themagnetic memory cells on that given word line are selected to be writtentogether at the same time. According to one exemplary embodiment, thereare 128 magnetic memory cells per word line.

In step 604, each of the magnetic memory cells selected in step 602,above, is written by sending a small current through each correspondingbit line (a bit line current). For example, if 128 magnetic memory cellsare selected in step 602 above, then a bit line current is sent througheach of the 128 corresponding bit lines to write data to those 128magnetic memory cells.

The bit line current can be either a positive current or a negativecurrent. A positive current will write a logic “1” to the correspondingmagnetic memory cell, and a negative current will write a logic “0” tothe corresponding magnetic memory cell.

In step 606, the word line current is removed. In step 608, the bit linecurrent is removed. As a result, data (i.e., either a logic “1” or alogic “0”) is written to each of the magnetic memory cells selected instep 602, above.

FIGS. 7A-B are graphs representing current pulse sequences used to writedata to an individual magnetic memory cell. Specifically, FIG. 7Aillustrates the current pulse sequence used to write a logic “0” to amagnetic memory cell, and FIG. 7B illustrates the current pulse sequenceused to write a logic “1” to a magnetic memory cell. The arrows in FIGS.7A and 7B represent the current pulses.

Referring to FIG. 7A, the sequence starts with the magnetic memory cellbeing in a zero applied magnetic field. As described in conjunction withthe description of FIG. 6, above, a current is first applied to the wordline, i.e., the word line is turned on. In this instance, a negativecurrent is then applied to the bit line, i.e., the bit line is turnedon. The current to the word line is then removed, i.e., the word line isturned off, followed by the current to the bit line being removed, i.e.,the bit line is turned off. As a result, a logic “0” will have beenwritten to the magnetic memory cell.

Referring to FIG. 7B, the sequence again starts with the magnetic memorycell being in a zero applied magnetic field. The word line is firstturned on, followed by the bit line. In this instance, however, apositive current is applied to the bit line. The word line is thenturned off, followed by the bit line being turned off. As a result, alogic “1” will have been written to the magnetic memory cell.

FIG. 8 is a graph illustrating critical switching curve 800 for magneticmemory cell 100. The values of H_(bit) and H_(word) are presented inOersteds (Oe). Switching curve 800 is analogous to the commonStoner-Wohlfarth astroid.

For magnetic fields inside curve 800, magnetic memory cell 100 isstable. For magnetic fields outside of curve 800, magnetic memory cell100 can be written.

Switching curve 800 was calculated with a single domain model using thefollowing parameters for magnetic memory cell 100. Magnetic layer 108has a thickness of six nanometers (nm); magnetic layer 112 has athickness of five nm; and magnetic layer 116 has a thickness of five nm.Each of magnetic layers 108, 112 and 116 has a M_(s) equal to 1,500electromagnetic units/cubic centimeter (emu/cc), an intrinsic anisotropyH_(i) of 25 Oe, and a diameter of 150 rm. The exchange coupling Jbetween magnetic layers 112 and 116 equals −0.05 erg/square centimeter(erg/cm²), wherein a negative J value denotes AP-coupling.

From switching curve 800 it is notable that the bit and word linefields, e.g., H_(bit) and H_(word), respectively, required to writemagnetic memory cell 100 are very small, for example, less than 50 Oefor the word line (which, as described above, may be distributed over128 magnetic memory cells per word line), and less than 10 Oe for thebit line. Thus, for an exemplary array having 128 magnetic memory cellsper word line, the total field per magnetic memory cell is 10+50/128˜10Oe.

By comparison, for toggle switching devices, such as those described inSavtchenko, one needs H_(bit)=H_(word)=80 Oe. Thus, the total field pertoggle switching device is 80+80/128=81 Oe. Therefore, magnetic memorycell 100 uses eight times less power than a typical toggle switchingdevice.

Another important feature of magnetic memory cell 100 is that whenH_(word) is zero, a very large bit line field is required to switchmagnetic memory cell 100. See, for example, switching curve 800 whereinan H_(bit) of greater than 300 Oe is needed to switch magnetic memorycell 100 when H_(word) is zero. Thus, magnetic memory cell 100 is verystable under half select. It is only when the word line is selected thatit is then easy to write magnetic memory cell 100 with a small bit linefield, as described above.

FIGS. 9A-D are graphs illustrating magnetic moment rotations in magneticlayers 108, 112 and 116 of magnetic memory cell 100. Specifically, inFIGS. 9A and 9B, the rotations of magnetic moments μ₁, μ₂, and μ₃ inmagnetic layers 108, 112 and 116, respectively, are depicted whenmagnetic memory cell 100 starts in a “0” logic state, and a logic “0” ora logic “1,” respectively, is written. In FIGS. 9C and 9D, the rotationsof magnetic moments μ₁, μ₂, and μ₃ in magnetic layers 108, 112 and 116,respectively, are depicted when magnetic memory cell 100 starts in a “1”logic state, and a logic “0” or a logic “1,” respectively, is written.In each of FIGS. 9A-D, μ₁ corresponding to magnetic layer 108 isdepicted as a line ending in a black circle, μ₂ corresponding tomagnetic layer 112 is depicted as a line ending in a gray circle and μ₃corresponding to magnetic layer 116 is depicted as a line ending in awhite circle. Circles have been used for ease of depiction andidentification of each magnetic moment with the corresponding magneticlayer. However, it is to be understood that each circle can be replacedwith an arrowhead, e.g., as in FIGS. 3A-B, described above wherein μ₁,μ₂, and μ₃ are represented by arrows 302, 304 and 306, respectively.

The graphs shown in FIGS. 9A-D were all calculated with a single domaintheory, using the same parameter values as in switching curve 800,above. Namely, magnetic layer 108 has a thickness of six nm; magneticlayer 112 has a thickness of five nm; and magnetic layer 116 has athickness of five nm. Each of magnetic layers 108, 112 and 116 has aM_(s) equal to 1,500 emu/cc, an H_(i) of 25 Oe, and a diameter of 150nm. Finally, the exchange coupling J between magnetic layers 112 and 116equals −0.05 erg/cm², wherein a negative J value denotes AP-coupling.

In FIG. 9A, as mentioned above, magnetic memory cell 100 starts in a “0”logic state, and a logic “0” is written. According to this exemplaryembodiment, μ₁ initially points to the left in the zero applied magneticfield. As described above, when μ₁ points to the left in a zero appliedmagnetic field, magnetic memory cell 100 registers a logic “0.”Beginning at the point of zero applied magnetic field, a current pulsesequence then occurs.

First, a 50 Oe word line field is applied. As the word line field isapplied, μ₁ rotates up to point along the hard axis direction. A smallnegative bit line field is then applied. This negative bit line fieldrotates μ₁ slightly to the left. As the word line field is thendecreased, the negative bit line field keeps μ₁ biased to the left, sothat when H_(word) has been decreased to zero again, μ₁ points to theleft. μ₁ stays pointing to the left as the bit line field is reduced tozero.

In FIG. 9B, as mentioned above, magnetic memory cell 100 starts in a “0”logic state, and a logic “1” is written. According to this exemplaryembodiment, μ₁ initially points to the left in the zero applied magneticfield. As described above, when μ₁ points to the left in a zero appliedmagnetic field, magnetic memory cell 100 registers a logic “0.” Asabove, beginning at the point of zero applied magnetic field, a currentpulse sequence then occurs.

First, a 50 Oe word line field is applied. As the word line field isapplied, μ₁ rotates up to point along the hard axis direction. A smallpositive bit line field is then applied. This positive bit line fieldrotates μ₁ slightly to the right. As the word line field is thendecreased, the positive bit line field keeps μ₁ biased to the right, sothat when H_(word) has been decreased to zero again, μ₁ points to theright. μ₁ stays pointing to the right as the bit line field is reducedto zero.

In FIG. 9C, as mentioned above, magnetic memory cell 100 starts in a “1”logic state, and a logic “0” is written. According to this exemplaryembodiment, μ₁ initially points to the right in a zero applied magneticfield. As described above, when μ₁ points to the right in a zero appliedmagnetic field, magnetic memory cell 100 registers a logic “1.” Asabove, beginning at the point of zero applied magnetic, a current pulsesequence then occurs.

First, a 50 Oe word line field is applied. As the word line field isapplied, μ₁ rotates up to point along the hard axis direction. A smallnegative bit line field is then applied. This negative bit line fieldrotates μ₁ slightly to the left. As the word line field is thendecreased, the negative bit line field keeps μ₁ biased to the left, sothat when H_(word) has been decreased to zero again, μ₁ points to theleft. μ₁ stays pointing to the left as the bit line field is reduced tozero.

In FIG. 9D, as mentioned above, magnetic memory cell 100 starts in a “1”logic state, and a logic “1” is written. According to this exemplaryembodiment, μ₁ initially points to the right in a zero applied magneticfield. As described above, when μ₁ points to the right in a zero appliedmagnetic field, magnetic memory cell 100 registers a logic “1.” Asabove, beginning at the point of zero applied magnetic field, a currentpulse sequence then occurs.

First, a 50 Oe word line field is applied. As the word line field isapplied, μ₁ rotates up to point along the hard axis direction. A smallpositive bit line field is then applied. This positive bit line fieldrotates μ₁ slightly to the right. As the word line field is thendecreased, the positive bit line field keeps μ₁ biased to the right, sothat when H_(word) has been decreased to zero again, μ₁ points to theright. μ₁ stays pointing to the right as the bit line field is reducedto zero.

In general, the three moments μ₁, μ₂, and μ₃ bodily rotate together,largely maintaining their original triangular relative orientation. Thepurpose of the AP-coupling between magnetic layers 112 and 116 is topromote this bodily rotation.

Thus, in conclusion, when magnetic memory cell 100 starts in either a“0” or “1” logic state, a word line field followed by a small negativebit line field will write a logic “0.” Similarly, when magnetic memorycell 100 starts in either a “0” or “1” logic state, a word line fieldfollowed by a small positive bit line field will write a logic “1.”

FIGS. 10A-B are graphs illustrating moment rotations for magnetic memorycell 100 when a field is applied in the easy axis direction.Specifically, FIG. 10A is a hysteresis graph illustrating magneticmoment μ₁ of magnetic layer 108 (normalized to 1) in the easy axisdirection and FIG. 10B is a graph illustrating the rotations of magneticmoments μ₁, μ₂, and μ₃ in magnetic layers 108, 112 and 116,respectively, when a field is applied in the easy axis direction. InFIG. 10A, the magnetic field applied by the easy axis (H_(easy)) isplotted on the x-axis and the component of μ₁ along the easy axis(normalized to 1) is plotted on the y-axis. In FIG. 10B, H_(easy) isagain plotted on the x-axis and the directions of the momentscorresponding to magnetic layers 108, 112 and 116 are shown. As withFIGS. 9A-D, described above, in FIG. 10B, μ₁ corresponding to magneticlayer 108 is depicted as a line ending in a black circle, μ₂corresponding to magnetic layer 112 is depicted as a line ending in agray circle and μ₃ corresponding to magnetic layer 116 is depicted as aline ending in a white circle. Circles have been used for ease ofdepiction and identification of each magnetic moment with thecorresponding magnetic layer. However, it is to be understood that eachcircle can be replaced with an arrowhead, e.g., as in FIGS. 3A-B,described above wherein μ₁, μ₂, and μ₃ are represented by arrows 302,304 and 306, respectively.

The progression of the magnetic moment rotations occurs in the sequenceindicated by arrow 1002. For example, starting at H_(easy) equals −400Oe and following the sequence indicated by arrow 1002, it is shown thatμ₁ starts pointing to the left, and subsequently flips to pointing tothe right, near +400 Oe, and then as the field is reversed the momentinitially stays pointing to the right and then flips to pointing to theleft near −400 Oe.

The graphs in FIGS. 10A-B were all calculated with a single domaintheory, using the same parameter values as in, for example, switchingcurve 800, above. Namely, magnetic layer 108 has a thickness of six nm;magnetic layer 112 has a thickness of five nm; and magnetic layer 116has a thickness of five nm. Each of magnetic layers 108, 112 and 116 hasa M_(s) equal to 1,500 emu/cc, an H_(i) of 25 Oe, and a diameter of 150nm. Finally, the exchange coupling J between magnetic layers 112 and 116equals −0.05 erg/cm², wherein a negative J value denotes AP-coupling.

As can be seen from FIGS. 10A-B, when a field is applied along the easyaxis, magnetic layer 108 does not respond initially, and magnetic layers112 and 116 become stabilized by the field. This is why such a largefield is required to switch magnetic memory cell 100 when the field isapplied along the easy axis.

FIGS. 11A-B are graphs illustrating moment rotations for magnetic memorycell 100 when a field is applied in the hard axis direction.Specifically, FIG. 11A is a hysteresis graph illustrating magneticmoment μ₁ of magnetic layer 108 (normalized to 1) in the hard axisdirection, and FIG. 11B is a graph illustrating the rotations ofmagnetic moments μ₁, μ₂, and μ₃ in magnetic layers 108, 112 and 116,respectively, when a field is applied in the hard axis direction. InFIG. 11A, the magnetic field applied by the hard axis (H_(hard)) isplotted on the x-axis and the component of μ₁ in the hard axis direction(normalized to 1) is plotted on the y-axis. In FIG. 11B, H_(hard) isagain plotted on the x-axis and the directions of the momentscorresponding to magnetic layers 108, 112 and 116 are shown. As withFIGS. 9A-D, described above, in FIG. 11B, μ₁ corresponding to magneticlayer 108 is depicted as a line ending in a black circle, μ₂corresponding to magnetic layer 112 is depicted as a line ending in agray circle and μ₃ corresponding to magnetic layer 116 is depicted as aline ending in a white circle. Circles have been used for ease ofdepiction and identification of each magnetic moment with thecorresponding magnetic layer. However, it is to be understood that eachcircle can be replaced with an arrowhead, e.g., as in FIGS. 3A-B,described above wherein μ₁, μ₂, and μ₃ are represented by arrows 302,304 and 306, respectively.

The progression of the magnetic moment rotations occurs in the sequenceindicated by arrow 1102. For example, starting at H_(hard) equals −50 Oeand following the sequence indicated by arrow 1102, it is shown that μ₁starts pointing down, and subsequently flips to pointing up, near +30Oe, and then as the field is reversed the moment initially stayspointing up and then flips to pointing down near −30 Oe.

The graphs in FIGS. 11A-B were all calculated with a single domaintheory, using the same parameter values as in, for example, switchingcurve 800, above. Namely, magnetic layer 108 has a thickness of six nm;magnetic layer 112 has a thickness of five nm; and magnetic layer 116has a thickness of five nm. Each of magnetic layers 108, 112 and 116 hasa M_(s) equal to 1,500 emu/cc, an H_(i) of 25 Oe, and a diameter of 150nm. Finally, the exchange coupling J between magnetic layers 112 and 116equals −0.05 erg/cm², wherein a negative J value denotes AP-coupling.

By way of comparison with FIGS. 10A-B, described above, as can be seenfrom FIGS. 11A-B, when a field is applied along the hard axis, moments302, 304 and 306 bodily rotate so that magnetic layer 108 points in thedirection of the applied field.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

1. A magnetic memory cell comprising: at least one fixed magnetic layer;and a plurality of free magnetic layers, separated from the at least onefixed magnetic layer by at least one barrier layer, comprising: a firstfree magnetic layer adjacent to the barrier layer; a second freemagnetic layer separated from the first free magnetic layer by at leastone spacer layer; and a third free magnetic layer separated from thesecond free magnetic layer by at least one anti-parallel coupling layer,wherein a magnetic moment of the first free magnetic layer is greaterthan both a magnetic moment of the second free magnetic layer and amagnetic moment of the third free magnetic layer.
 2. The magnetic memorycell of claim 1, wherein the at least one spacer layer is configured toprevent coupling between the first free magnetic layer and the secondfree magnetic layer.
 3. The magnetic memory cell of claim 1, wherein theat least one anti-parallel coupling layer is configured to mediateanti-parallel coupling between the second free magnetic layer and thethird free magnetic layer.
 4. The magnetic memory cell of claim 1,wherein the magnetic moment of the second free magnetic layer equals themagnetic moment of the third free magnetic layer.
 5. The magnetic memorycell of claim 1, wherein at least one of the first free magnetic layer,the second free magnetic layer and the third free magnetic layercomprise one or more of a nickel/iron alloy and alloys containing one ormore of nickel, iron, cobalt and a rare earth element.
 6. The magneticmemory cell of claim 1, wherein the first free magnetic layer, thesecond free magnetic layer and the third free magnetic layer have a samecomposition.
 7. The magnetic memory cell of claim 1, wherein the atleast one spacer layer comprises one or more of tantalum nitride,tantalum, titanium, tungsten and niobium.
 8. The magnetic memory cell ofclaim 1, wherein the at least one anti-parallel coupling layer comprisesone or more of ruthenium, osmium, copper, chromium, molybdenum, rhodium,rhenium and iridium.
 9. The magnetic memory cell of claim 1, wherein arelative orientation of the magnetic moment of the first free magneticlayer, the magnetic moment of the second free magnetic layer and themagnetic moment of the third free magnetic layer is triangular in zeroapplied field.
 10. The magnetic memory cell of claim 1, wherein the atleast one fixed magnetic layer comprises: an anti-ferromagnetic layer; afirst fixed magnetic layer adjacent to the anti-ferromagnetic layer; anda second fixed magnetic layer separated from the first fixed magneticlayer by at least one anti-parallel coupling layer.
 11. The magneticmemory cell of claim 10, wherein the anti-ferromagnetic layer comprisesone or more of a metal alloy, a platinum/manganese alloy and aniridium/manganese alloy.
 12. A magnetic random access memory device,comprising: a plurality of word lines oriented orthogonal to a pluralityof bit lines; and a plurality of magnetic memory cells configured in anarray between the word lines and bit lines, at least one of theplurality of magnetic memory cells comprising: at least one fixedmagnetic layer; and a plurality of free magnetic layers, separated fromthe at least one fixed magnetic layer by at least one barrier layer,comprising: a first free magnetic layer adjacent to the barrier layer; asecond free magnetic layer separated from the first free magnetic layerby at least one spacer layer; and a third free magnetic layer separatedfrom the second free magnetic layer by at least one anti-parallelcoupling layer, wherein a magnetic moment of the first free magneticlayer is greater than both a magnetic moment of the second free magneticlayer and a magnetic moment of the third free magnetic layer.
 13. Themagnetic random access memory device of claim 12, wherein the first freemagnetic layer has an intrinsic anisotropy set along a direction ofmagnetic field applied by the bit lines.